When information is transmitted over a wireless channel then the received signal can be distorted due to the effects of multipath transmissions. Multipath transmissions can be caused by reflections of the transmitted signal off of objects such as buildings, vehicles, and other types of obstructions. The result is that multiple copies of the transmitted signal arrive at the receiver, with each signal having a certain time delay. The amount of the time delay is proportional to the length of the path the signal traveled. Obviously, signals that are reflected multiple times will travel a longer path than a signal with a direct line of sight.
The amount of time between a first signal (typically the line of sight signal) and a last reflected signal arrive at the receiver is referred to as the delay spread. In cellular communication applications delay spreads can be on the order of several microseconds. The delay induced by multipath can cause a symbol received along a delayed path to “bleed” into a subsequent symbol arriving at the receiver via a more direct path. This is typically referred to as inter-symbol interference (ISI). In a single carrier communication system, the symbol times decrease as the data rates increase. At very high data rates (with correspondingly shorter symbol periods) it is possible for ISI to exceed an entire symbol period and spill into a second or third subsequent symbol. This can cause significant problems at the receiver.
In addition to problems in the time domain, there can be additional problems in the frequency domain due to multipath distortion. As the multiple reflected signals are combined at the receiver, some frequencies within the signal passband can undergo constructive interference (a linear summation of in-phase signals), while other signals may undergo destructive interference (a linear summation of out-of-phase signals). This can result in a composite received signal that is distorted by frequency selective fading. The ability to estimate signal power and noise power for a received signal can significantly improve the ability to receive a signal that has distortion.
Reference will now be made to the exemplary embodiments illustrated, and specific language will be used herein to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended.